welcome bkrell, to the iris forum. wish i could help start the search, but you will need to take close-ups of all parts of the flower and plant. you might also want to include the height of the iris, size of the flowers, color of the beards, and whether your iris may have purple-base foliage.
there are so many look-alike irises, and unless you grow all of them side-by-side there maybe no conclusive evidence....it's a tough one, but maybe someone who knows more about historical irises might be able to help you? there may be fewer historical blue iris, so it may be easier to identify?
if the task might be too daunting, your iris is beautiful iregardless of a name. enjoy and love it for the garden legacy it brings with it. it was loved by your great-grandmother, and has found it's way to you to be loved.
if you stick around long enough on this forum, i am sure that there are many here that would share a yardful of beautiful, named iris for the cost of postage.
give the one you have a lot of love.....it is very pretty!